Method of forming corrosion resistant coating on metallic surfaces with ferrate solutions



United States Patent O METHOD OF FORMING CORROSION RESISTANT COATING ON METALLIC SURFACES WITH FER- RATE SOLUTIONS John Ruflford Harrison, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware 5 Claims. (Cl. 148-614) No Drawing.

This invention relates to an improved method of ap- "plying ferrates to metallic surfaces to protect the latter against corrosion.

In application Serial No. 457,574, filed September 21, 1954, by John E. Castle, of common assignment herewith, there is disclosed a method of protecting metallic surfaces by contacting them with alkaline solutions of alkali metal ferrates. The method of the co-pending application will be referred to in this specification as the normal" ferrate treatment. Although the normal method is applicable primarily to ferrous materials such as cast iron and steel, it may also be employed with other commercial metals such as zinc.

In the normal ferrate treatment the workpiece to be treated is generally soaked for about one hour in an alkaline ferrate solution at ambient temperature. The solution of ferrate should be of about 1% concentration for best results and may be buttered at a pH of at least around 11 to prevent decomposition of the latter. Carbonate-bicarbonate buffers are conveniently employed to maintain this pH at a concentration of around one mole with respect to each buffer constituent. The procedure described builds up a coating or layer on the metallic surface which prevents further corrosion of the metal. This coating may be termed a ferrate film although its exact chemical composition is unknown.

The ferrate film is effective itself in discouraging corrosion and is, perhaps, even more useful as a substrate upon which other chemical treatments can be used. The value of the process is somewhat impaired, however, by the length of time required to accomplish it. The primary object of this invention is, consequently, provision of a method whereby ferrate films can be produced on metallis surfaces more rapidly than has been possible heretofore.

It has been found that the process of depositing ferrate films on metallic surfaces can be greatly expedited if the surface is treated, before contact with the ferrate solution is made, in an aqueous solution of certain compounds capable of decomposing ferrates. Usable compounds are, in general, reducing agents of which 8-hy' droxyquinoline is the best.

Usually the workpiece to be treated is given a pre treatment consisting of a pickle in concentrated hydrochloric acid, a rinse and a neutralization in dilute caustic containing some cyanide. For the purposes of this invention, 8-hydroxyquin'oline is most conveniently applied during the pretreatment in the solution used for neutralization, the cyanide being omitted. The 8-hydroxyquinoline can alternatively be applied to the metallic surface by dipping the workpiece in a separate aqueous solution.

During the contact between S-hydroxyquinoline and iron a green deposit forms on the surface of the latter. This deposit, of unknown composition, appears to be essential for acceleration of the subsequent ferrate treatment. Its formation is quickened by increasing the temperature.

'bath containing around 0.4% of 8-hydroxyquinoline and 12 /2% sodium hydroxide. When a workpiece is treated with 8-hydroxyquinoline as described, subsequent contact with a ferrate solution for 10-20 minutes will give as good protection as a contact time of several hours if the workpiece is not so treated.

Other reducing agents of value in expediting the production of a ferrate film are thiocyanates, calcium and sodium thiocyanates in particular. These salts are utilized in substantially the same manner as is S-hydroxyquinoline. Around 10% by weight of an alkali or alkaline earth thiocyanate is optimum although 1-20% can be used.

Details of this invention will be evident from the examples which follow. In these examples all percentages are by weight unless otherwise noted.

Example 1 This example shows the production of a normal ferrate film as shown in the aforementioned co-pending application and is included solely as a control.

A steel strip was pickled, neutralized in alkaline cyanide and then partially given a protective ferrate film by the immersion of one inch of it in a 1% potassium ferrate solution of pH 11 for one hour at ambient temperature. and placed in the closed atmosphere above an acidified aqueous solution containing sodium pyrosulfite, NazSzOs. It was maintained at 40 C. in the corrosive sulfur dioxide-containing atmosphere for around 20 hours. Only slight corrosion was evident on the treated part of the strip at the end of the exposure. The value of the ferrate solution is almost directly proportional to the contact time between it and the workpiece at contact times less than one hour in duration.

Example 2 This example shows the effect of S-hydroxyquinoline in speeding up the ferrate treatment.

(a) A steel strip was pickled in concentrated hydrochloric acid and rinsed. It was then dipped for three minutes into a bath containing 0.4% 8-hydroxyquinoline and 12.5% of sodium hydroxide and maintained at around C. The strip was withdrawn from the bath, allowed to drain for about one minute and immersed in a ferrate bath of the type shown above. After an immersion of only 10 minutes the strip was withdrawn and tested for corrosion resistivity as in the first example. This strip, soaked for only 10 minutes, was found to have as great a resistance to corrosion as that tested after a normal ferrate treatment of one hour.

(b) Steel strips were pickled in concentrated hydrochloric acid and rinsed. They were then held separately for 3 minutes in baths containing aqueous alkaline solutions of 8-hydroxyquinoline of difierent concentrations and pHs at 90 C. After being allowed to drain for a minute they were immersed for 10 minutes in a ferrate bath of the type of Example 1 and rinsed. The best protection was found at a concentration of S-hydroxyquinoline of 0.4% and a concentration of sodium hydroxide of 12.5%. When the concentration of 8-hydroxyquinoline was raised to 1.25% and that of the NaOH lowered to this value, a thin protective film only was formed.

The strip was then withdrawn, rinsed and dried Example 3 This example shows the use of a very stable ferrate solution after a pretreatment with 8-hydroxyquinoline.

Two steel strips were treated with 8-hydroxyquinoline in the manner of Example 2(a). One strip was then dipped into a ferrate solution of the type shown by Ex ample 2(a) while the other was immersed in a 1% solution of ferrate in 10 N NaOH. Ferrate films of equal corrosion resistance formed on each in 10 minutes While a normal ferrate treatment at pH 11 requires only about one hour, ferrate dissolved in 10 N NaOH needs at least 4 hours to provide a suitable coating. The distinction in treatment time thus disappears after the immersion in 8-hydroxyquinoline. Use of a ferratc solution containing 10 N NaOH is preferred because the ferrate in such a solution is stable for several days. At a pH of 11 a ferrate is stable for only an hour or two.

Additional experiments showed that in the 10 N solution the ferrate concentration for use in conjunction with an S-hydroxyquinoline pretreatment can be lowered to 0.3% with no loss of activity.

Example 4 This example shows the effect of calcium thiocyanate in accelerating the ferrate-treatment.

Six steel samples were pickled in hydrochloric acid, rinsed and neutralized with an alkaline cyanide solution. After being rinsed again they were immersed for ten minutes in calcium thiocyanate solutions of the respective concentrations of 1, 5, 10, 20, 30 and 55%. They were then dried in a stream of air and immersed for twenty minutes in a 1% solution of ferrate buffered at pH 11. The ferrate film produced on the sample treated in the 10% thiocyanate solution exhibited good corrosion resistance. Results obtained with the 5% thiocyanate solution were almost as good while those obtained with the 1% and 2% were but slightly worse. The two higher concentrations gave loose, patchy films.

Later experiments showed that the period of thiocyanate treatment could be reduced to one second or less and that of the subsequent ferrate treatment to 10 minutes without reduction in the corrosion-resistance of the resulting film.

Example 5 This example shows the eifect of sodium thiocyanate in accelerating the ferrate treatment.

Steel samples were pickled, rinsed and then immersed for ten minutes in solutions containing various proportions of sodium thiocyanate and sodium hydroxide. They were then air-dried and dipped for 10 minutes in a 1% solution of ferrate buffered at pH 11. The optimum thiocyanate concentration was found to be 10% and independent of the hydroxide concentration.

The ferrate films initially produced were not quite as good as those from a calcium thiocyanate treatment. Later work demonstrated that the films could be brought up to the standard of a normal one-hour ferrate treatment by increasing the time of immersion in the ferrate solution to 15 minutes.

Having described my invention, I claim:

1. The method of protecting a ferrous metal such as cast iron, steel or the like against corrosion which comprises contacting said metal successively with a first solution containing a reducing agent selected from the group consisting of alkali and alkaline earth metal thiocyanates and 8-hydroxyquinoline and then with a second solution containing an alkali metal ferrate.

2. The method of claim 1 in which the first solution contains around 0.34% by weight of the S-hydroxyquinoline.

3. The method of claim 2 in which the contact time between the metal and the second solution is around 10-20 minutes.

4. The method of claim 1 in which the first solution contains around 120% by weight of calcium thiocyanate.

5. The method of claim 1 in which the first solution contains around l-2.0% by weight of sodium thiocyanate.

No references cited. 

1. THE METHOD OF PROTECTING A FERROUS METAL SUCH AS CAST IRON, STEEL OR THE LIKE AGAINST CORROSION WHICH COMPRISES CONTACTING SAID METAL SUCCESSIVELY WITH A FIRST SOLUTION CONTAINING A REDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METAL THIOCYANATES AND 8-HYDROXYQUINOLINE AND THEN WITH A SECOND SOLUTION CONTAINING AN ALKALI METAL FERRATE. 